DNA damage accumulates during life and is thought to contribute to aging and genomic instability. Therefore, defining those proteins and pathways that maintain genomic stability is critical in preventing aging and age-related degeneration. This project aims to understand what roles the five human RecQ proteins play in DNA repair and genomic stability. RecQ proteins play fundamental roles in several DNA metabolic pathways including DNA double-strand break repair (DSBR), mainly in the form of homologous recombination (HR), non-homologous endjoining (NHEJ) and replication. One major goal of our lab is to delineate the unique and complementary roles of the human RecQ helicases, with the expectation that we might fully explain the function of each protein. Each RecQ possesses helicase and strand annealing activity as well as domains that confer unique functionality. We have demonstrated that all five human RecQ helicases can be recruited to laser-induced double strand breaks (DSB), however the details of how each RecQ participates in DSB is less clear and therefore efforts are ongoing to dissect more precisely the role of each RecQ in DSB repair. RECQL4 is highly expressed during S phase when HR-dependent DSBR is active. Consistent with this, we found that RECQL4-deficient cells have a profound defect in HR-mediated DSBR. We showed that RECQL4 interacts with the MRN/CtIP complex, and that its helicase activity is required for DNA end-resection during DSBR. We are continuing to investigate the interactions of RECQL4 with other DSBR proteins. One abundant interaction partner for RecQs that is of interest is Poly(ADP-ribose) polymerase (PARP1). PARP1 and the polyADP-ribose polymer (PAR) play important roles in the response to DNA damage. PARP1 is a NAD+-dependent DNA damage-sensor that covalently attaches multiple ADP-ribose moieties onto its protein substrates, including itself. Our experiments suggest that PARP1 and/or PAR play a role in recruiting WRN and RECQL5 to DSBs, as their recruitment is delayed in PARP1-deficient cells or in cells treated with PARP1 inhibitors. We also found that PAR stimulated the strand annealing (SA) activity of RECQL5, but not that of WRN. In contrast, PARP1 and PARylated PARP1 inhibited the helicase and SA activities of WRN and RECQL5. PARP1 stimulated the SA activity and inhibited the helicase activity of RECQL4. All RecQ proteins possess PAR binding motifs (PBMs), but little is known about these putative PBM-PAR interactions, except for in WRN. PARP1 is thought to play an important role in alternative NHEJ, therefore we are investigating any functional interaction between RECQL4 and PARP1 that ultimately may help promote this repair mechanism. Our findings have implications on how cells decide which DSBR subpathway is active at any given DSB (i.e., pathway choice) thus we continuing to characterize the role of RecQ proteins in DSBR pathway choice.